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Parablepsis comes from a Greek word meaning "looking by the side." It is a technical term used in palaeography (the study of ancient handwriting), papyrology (the study of documents written on papyrus), and codicology (the study of ancient codices) to explain why certain ancient and medieval manuscripts seem to have words missing.

In a time before the invention of the printing press, all books were copied by hand. A scribe would keep the book he was copying (called the exemplar) on the table nearby as he worked on a fresh copy. However, fatigue, eyestrain, boredom, or incompetence might cause the scribe to lose track of what he was doing and "drop" a few lines over the course of his writing session.

This phenomenon is easiest to illustrate with an example. Here is a famous case of parablepsis drawn from the study of the New Testament.

The Greek text of John 17:15 looks something like this.

ουκ ερωτω ινα αρης αυτους εκ του
κοσμου αλλ ινα τηρησης αυτους εκ του
πονηρου.

The English translation, formatted the same way, would read as follows:

I am not asking you to take them from the
world, but I ask you to protect them from the
evil one.

Note that two lines end with exactly the same words, which I have put into boldface type. A situation in which separate lines of a document end with the same words is called homoeoteleuton ("similar endings").

In one early New Testament manuscript, a scribe seems to have let his eye drift from the first instance of autous ek tou ("them from the") to the second instance as he glanced back and forth between his exemplar and the copy of it that he was making. This parablepsis caused him to inadvertently omit much of the second line of the passage as he jumped directly from the first line to the third. In English, what he ended up writing reads as follows:

I am not asking you to take them from the
evil one.

This garbled version of the sentence doesn't make much sense -- but the existence of other manuscripts, containing the complete text, makes the scribal error clear.

A scribe's inadvertent omission of words -- or sometimes even entire sentences -- is called haplography, and it is not always caused by parablepsis though it frequently is.